Decades of research have linked stress to adverse physical and emotional health outcomes in aging adults. More recently, studies have identified stress-related variables as important predictors of cognitive aging. However, there is considerable variability in the negative effects of stress, both within and across individuals. Characteristics of different stressors contribute to this variation, but individuals exposed to the same or similar stressors still exhibit substantial differences in their responses. Understanding the mechanisms that can account for this variability represents a critical challenge for explaining how stress affects cognitive health as well as many other aging-related physical and emotional health outcomes. A person's tendencies to recurrently think about and mentally 'relive' problematic situations and events can amplify and extend emotional and physiological responses, even after cessation of the eliciting stressor. We hypothesize that such unconstructive repetitive thought (URT), which encompasses concepts such as worry and rumination, operates as a final psychological pathway by which stressors exert their harmful effects on cognitive health. We propose to test this hypothesis by conducting a longitudinal investigation of URT as a potential mediator of the effects of stress (current and cumulative) on cognitive function. Because early indicators of cognitive aging manifest long before old age, the proposed sample will consist of 320 racially diverse individuals in early adulthood and midlife (ages 25-65). Our design will be a 'measurement-burst', consisting of 8 biannual 'bursts' of 14 daily assessments of cognition, stress, URT, affect and physiological markers. This design will allow modeling of lead-lag relationships among these variables across time-scales that range from days to years. This study will address four aims: Aims 1 and 2 examine the role of URT in accounting for the effects of current stress on short-term (daily) and mid-term (biannual) in traindividual cognitive variability and change. Aims 3 and 4 examine URT as a mediator of the cumulative effects of stress on long-term (across years) cognitive change. This significance of this study lies in: 1) improved understanding of how stress in early adulthood and midlife affects risk for cognitive aging; 2) identifying a mechanism (URT) that links the experience of stress to cognitive aging; 3) resolving inconsistencies regarding the time course relating stress, HPA axis function and cognition; and 4) determining the temporal ordering of stress and cognitive change. Areas of conceptual and methodological innovation include: (1) a novel theoretical framework that can explain both acute and chronic stress effects on cognition, with important implications for a broad range of mental and physical health outcomes; (2) use of a measurement burst design to evaluate mediational hypotheses by examining lead-lag relationships across different time scales ranging from days to years; and (3) use of an analytic approach to examine both in train individual and in train individual facets of cognitive change.